It should start slowly and smoothly accelerate to a hard stop on both backcast and forecast. Acceleration should not suddenly increase just before the time of the stop. It needs to be a smoothly increasing acceleration from beginning to the end at the stop. Think in terms of hammering 2 nails with a two headed hammer that it driving the nail in boards about 2 feet apart with both the foreward and backward motions of the hammer hitting one of the nail.